"the motor" is a vast and complex item, to say that "the motor" is the problem doesn't give us enough information to tell you what it should cost to fix it. You may have something as simple as a wiring or carb issue, you may need a new piston and bore/hone, you may need a top end rebuild (piston/cam/valves/springs/seats/etc) or you may need a new transmission. You may need a new clutch, you may simply need to change/drain/add oil.
What are the symptoms of your problem, perhaps we can help you narrow things down.
If there's no gas getting into the head I'd start with the gas line and petcock. And once you've verified the gas is coming out of the tank check out the carb, clean and inspect the float/needlejet/jetneedle/diaphram/etc.
As you can see, there's more than a few parts in here that could be blocking that fuel from getting from the carb side of the petcock to the head.
I dropped it off at a yamaha dealer and it turns out that the guy i bought it from messed up the timing and droppeed a nut next to the valves and the valves shot it straight thru the cam and it needed a new exaughst. It came to only 243 dollars with new cam and high performance exhaust.